The present invention relates to an optical disk in which a recording layer and a dielectric layer are formed on a transparent substrate to rewrite information based on a rise in temperature generated by irradiation of laser beam and a method of manufacturing optical disk.
Recently, in the field of recording medium or the like used as an external memory of computers, as an amount of information processed by computers or the like increases, there is an increasing demand for increasing a recording capacity of a recording medium used as an external memory.
Further, there is an increasing demand that a recording medium can cope with an improvement of capability of central processing unit (CPU) of computer and a variety of applications.
Therefore, it is very effective to use an optical disk capable of very high-density recording and which is excellent in portability as a recording medium for an external memory of large storage capacity. In particular, the use of rewritable optical disks effectively utilizing a magnetooptical effect or a crystal--amorphous phase transition phenomenon is very promising.
Under the above-mentioned situations, a demand for making the optical disks become capable of high-density recording increases more and more. In order to improve a recording density, a wavelength of laser beam is shortened, and a so-called mark edge system is employed.
It is requested that optical disks used as external memory of computer are able to record information in a recording power of wide range so as to prevent recording/reproducing characteristics from being affected due to various factors such as a difference of disk drives, lens smudged by dusts or the like or a difference of environmental temperature.
However, in optical disks capable of high-density recording by laser beam with a short wavelength and the mark-edge recording system, a margin of recording power in which a satisfactory recording is possible is reduced due to deterioration of S/N (signal-to-noise ratio) and C/N (carrier-to-noise ratio) or thermal interference generated between adjacent codes. In particular, since the optical disk becomes capable of high-density recording, a recording mark length is fluctuated considerably due to an influence (thermal interference) of heat generated by mark immediately before the recording.
In order to solve these problems, it is necessary to control a flow of heat generated immediately before the recording. Specifically, a recording layer should have a temperature characteristic which is easily heated and cooled when the recording layer is irradiated with laser beam upon recording. The optical disk requires such temperature characteristic as a characteristic for suppressing thermal interference.
In order to solve the aforesaid problems, there are previously-proposed methods for controlling shape and size of a unit recording region (e.g., recording magnetic domain in a magnetooptical recording medium) by controlling flow of heat within the recording medium, e.g., the following methods (1) to (4):
(1) To control a thermal conductivity of a reflecting film (see Japanese laid-open patent publication No. 2-152050); PA1 (2) To control a thermal conductivity of a protecting layer (UV protecting layer) (see Japanese laid-open patent publication NO. 4-337545 and Japanese laid-open patent publication No. 2-240846); PA1 (3) To provide two reflecting films thereby to control a thermal conductivity of the reflecting films (see Japanese laid-open patent publication NO. 5-342655); and PA1 (4) To deposit a dielectric film on the reflecting film (see Japanese laid-open patent publication No. 3-105742).
However, the aforesaid previously-proposed methods (1) to (4) can improve any one of a characteristic in which a recording layer tends to be heated quickly, i.e., a temperature rise characteristic and a characteristic in which a recording layer tends to be cooled quickly, i.e., a temperature fall characteristic and cannot satisfy both of the temperature rise characteristic and the temperature fall characteristic.